1. Field of the Invention
The invention relates to a method and apparatus for obtaining desired water-deficit stress levels in a plant by managing plant temperature through irrigation control. Plant temperature is correlated with the level of water-deficit stress. Desired water-deficit stress levels can be obtained by managing plant temperatures through an interactive plant temperature monitoring and irrigation dispensing system.
2. Description of the Prior Art
Plants, as sedentary poikilotherms, are subject to thermal variation. Environmental temperatures vary according to both diurnal and seasonal patterns. The thermal environment of many temperate plants is bounded by lethally low temperatures that define their growing seasons. Within the non-lethal thermal range, the plant is subjected to a continuously variable thermal environment. Temperature plays an important role in plant environment interactions and is perhaps one of the most pervasive influences on plant growth and development. Environmental temperature has been used to predict and explain plant growth and development in terms of heat units or growing degree-day approaches.
The relationship between plant temperature and environmental temperature is potentially rather complex. In the literature plant temperature is often assumed to be similar to the air temperature though it is generally acknowledged that under water deficits the temperature of the plant can be higher than that of the air. Direct measurement of plant temperature using contact thermometers and thermistors is possible though often time consuming and tedious. Non-contact thermal measurements using infrared thermometers have become increasingly common with advances in field. Lower cost infrared thermometers are now available for use in production agricultural settings. The temperature of plant and crop canopies can now be measured near-continuously over seasonal time scales.
The concept of thermal optimality of organisms is well documented. Biochemical reactions are inherently thermally dependent with reaction rates relatively sensitive to temperature. Given that the temperature of a plant is related to that of its environment, and that the temperature of the environment is constantly variable, the rates of the biochemical reactions of the plant are continuously affected by temperature in a potentially complex manner. The concept of a metabolically optimal state, while perhaps obvious in principle, is potentially complex in definition.
In the simplest sense an optimal metabolic state in a plant is the metabolic condition apparent in the plant when it is functioning in the absence of external factors that limit its performance. In the more common parlance it refers to a non-stressed condition. While the existence of a non-stressed, or optimal condition, of a plant is recognized as an ideal, the reality is that plants often function under external constraints.
In an agricultural crop, the optimal metabolic state may not be the same as the desired agricultural state. Such a desired agricultural state is defined anthropomorphically in terms of the desired agronomic outcome defined with respect to the agricultural product. Issues of yield and quality are central to the desired agronomic outcome.
In a forage crop, total seasonal biomass may be the desired agronomic outcome while in an oilseed crop the yield and quality of the oil may be used to define a desired agronomic outcome. In cotton, biomass, a result of optimal metabolism, is of value only to the extent to which it is related to the desired agronomic outcome defined in terms of fiber yield and quality. Fully optimized vegetative growth in cotton is often undesirable as it is associated with reduced harvest index and negative fiber characteristics.
In an organism that is subject to thermal variation, the rate of enzyme reactions will vary continuously. Increases in temperature are known to increase plant tissue respiration, as exemplified by a study of soybean (Glycine max) leaves that showed that respiration increased by a factor of 2.5 between 18° C. and 26° C. average night temperatures [Bunce. 2005. Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions. Ann Bot (Lond) 95: 1059-1066]. It has been demonstrated that the thermal dependence of the kinetics of enzymes can be used to define biologically optimal temperatures of crops. Irrigation management based upon thermal optima defined in terms of enzyme kinetic properties has proven to optimize cotton production. Kinetic properties of enzymes responsible for herbicide activity have been used to explain thermal dependency of some aspects of herbicide efficacy. This approach was used to define optimal thermal ranges for herbicide efficacy. The thermal dependence of kinetics of malate synthase from cotton was used to develop a model that predicts optimal cotton emergence under thermal variation.
It is well established that irrigation based upon canopy temperature measurements is capable of altering the relationship between the temperature of the plant canopy and the plant's thermal environment. In the most general sense, as plant water use increases canopy temperature decreases. The canopy temperatures of plants experiencing water deficits are generally elevated relative to those that are well-watered. This relationship provides the basis for the BIOTIC irrigation management protocol (biologically identified optimal temperature interactive console) that is designed to maintain crop water status in an optimal condition (Upchurch et al., U.S. Pat. No. 5,539,637). Such an approach serves to prevent non-optimal water and metabolic states in the crop. The temperature of a plant indicates the water status of the plant and, when compared to a biologically-based indicator of optimality, a measure of optimal and non-optimal metabolic status.
However, despite these and other advances, the need remains for improved irrigation control systems which are capable of maximizing product quality, and which are suitable for use in environments where water availability is limited.